1. Field of the Invention
This invention relates to printing presses and more particularly, to an improved offset printing press, which includes a combination plate, blanket and imaging cylinder for holding an image formed on a thin film member.
2. Description of the Prior Art
The art of printing has been around since at least 1447 when Gutenberg first printed the bible using moveable type. Since Gutenberg first invented a printing system with movable type, four major developments have occurred to bring the printing industry to its modern place in society. First, the composing machine permitted the mechanical assembly of a page much quicker than Gutenberg and his successors could do by hand. Next, the application of power to printing presses permitted development of the modern high speed, web and sheet fed, multicolor rotary presses. The third significant improvement was the application of photography to printing, first to photo-engraving and later to lithography which simplified prepress operations. The last significant improvement, which is relatively recent, is the application of electronic computers to compose pages to be printed. Computer composition includes automatic word processing and page assembly with graphics and halftones.
The utilization of computer page composition, however, has not been easily transferred to the modern high speed, web and sheet fed, multicolor rotary press. Generally, printing plates for web or sheet fed offset presses are prepared by exposing the photosensitive surface of a printing plate to a source of actinic radiation while the plate is in contact with a film negative. The film negative acts as a stencil, only allowing the plate to receive radiation in the image areas. After exposure, the plate is chemically treated to develop separate ink and water receptive image areas. In modern printing establishments, the film image may be exposed by a laser typesetter, which device transports the film past a rapidly scanned laser beam so as to receive a raster image generated with a computer or derived from an input scanner.
Modern printing processes include (1) relief printing, where the raised surface on a printing plate carries the ink and defines the information to be printed; (2) planographic printing, such as an offset printing press, where the printing surface is essentially flat and the printing plate is chemically treated to be separated into ink receptive (hydrophobic) and water receptive (hydrophilic) image areas; and (3) gravure printing, where an engraved or etched printing plate is used and ink is scraped from the raised surfaces, and only the etched printing plate surfaces result in ink transfer. Printing processes, which are not applicable to this invention, include silk screening, gravure and flexographic relief printing.
The subject invention relates primarily to an improvement in the planographic, or offset, printing process. This process makes use of the fact that certain substances are hydrophobic, that is repel water, such as wax, grease, and certain types of polymers, while other substances are hydrophilic, that is accept water, such as aluminum, zinc, chromium and other metals. In printing, ink is more like a grease and adheres to those areas which have not accepted the water. In its simplest form, the offset process includes preparing an image on a printing plate, where selected areas of the printing plate will hold water, or other dampening solutions, but the image to be printed repels the water and holds the ink. Next, both the image and non-image surfaces are dampened, but the image surface rejects the water. Then, both the image and non-image surfaces are inked, but only the imaged surface holds the ink. Lastly, the ink is transferred to the paper, or other media by direct contact.
In the offset process, the image may be indirectly applied to the media through an intermediate transfer, or blanket cylinder, whereby the image from the plate is applied first to a blanket cylinder and then, from the blanket cylinder to the media. Heretofore, the direct transfer of an image from a plate has been used only sparingly, generally for making lithographic prints, such as of a painting; high speed printing applications all use the offset printing process. To obtain quality print at high speed, it is necessary to have hard surfaces contact soft surfaces in order to accommodate surface irregularities and the intermediate blanket cylinder provides a soft surface between the hard plate and hard media.
Thus, a typical modern offset printing press includes three cylinders, which are the plate cylinder, for holding the imaged printing plate, the blanket cylinder, which is generally a metal cylinder with a blanket, which blanket is a composite of open or closed cell layers for compliance and web layers for dimensional stability, with a compliant surface layer to accept the inked image, and the impression cylinder for carrying the paper, or other media, to be printed. In addition, one or more additional cylinders, may typically be used to guide the paper to the desired position and are referred to generally as the delivery, transfer or transport system. The printing plate is imaged and processed by known techniques, such that the image to be printed holds the ink and repels the water. The printing plate is then affixed to the plate cylinder. The plate cylinder has a pair of additional systems, that is the fountain system and the inking system, for respectively moistening the printing plate and adding the ink to the imaged portion thereof. The ink image is then transferred to the blanket cylinder, and from the blanket cylinder, the ink image is transferred to the media.
One of the problems with offset printing plates is that they are not sufficiently compliant to permit printing a quality image directly on the hard paper media. Thus, as previously noted, an intermediate compliant surface blanket cylinder is required. If one could develop a printing plate which is sufficiently compliant, which at the same time maintains dimensional stability for image registration, so as to permit quality printing, the intermediate blanket cylinder could be eliminated. Such a printing plate could then be mounted to a compliant material on the plate cylinder to provide a compliant surface carrying the ink to directly contact the hard media to be printed. Such a system would not only eliminate the cost of the blanket cylinder, but would additionally reduce the loss of print quality resulting from the double transfer of the image, first to the blanket cylinder and then to the paper.
Another problem in the prior art has been the manner in which the printing plate is imaged. Generally, imaging requires starting with the image to be reproduced, making a negative thereof, and chemically reproducing that image on the printing plate. The process is quite expensive, labor intensive and time consuming. Modern computer systems permit composing entire pages directly on a computer screen, including text, graphical and half tone presentation of information. However, these signals still cannot be provided directly to the printing press; they first must be sent to a composing room to prepare an intermediate film which, in turn, is used to prepare a printing plate. It would be advantageous to permit the signals defining the image to expose a plate directly on the press, previously preloaded with a blank printing plate. Such a direct process of plate preparation would make the step of imaging much less expensive, much quicker and much less prone to distortion due to chemical processing and physical handling of the printing plates, and when used with multiple separation color images, the direct process of plate preparation permits electronic registration to be utilized.
In printing, publications, such as newspapers and magazines, which require a large number of copies and have a fixed format, are printed on high speed rotary web presses. Most publications, however, require less than 10,000 impressions and these short run publications are generally printed on sheet fed presses and duplicators. When color is involved, plate preparation for both types of presses is similar. Plates are prepared separately from the press and are transported to and mounted on the press. These plates must be robust to maintain dimensional stability of the image while handled. If the plates were fixed to the press, forming a composite structure, and then imaged, the plates could be very thin films since they are not transported. Further, a thin film is a compliant member. If instead of fixing the plate to a hard cylinder, a thin film plate were fixed to and imaged on the blanket cylinder, the plate cylinder could be eliminated. With a very thin film plate fixed to its surface, the blanket cylinder retains all of its former compliant attributes. With the plate cylinders removed from a four color press, the press architecture can be re-arranged to combine the function of plate preparation and printing in one system. Further, these functions can be automated so that plate preparation occurs while printing. This arrangement leads to a high productivity, fully automated, on demand printing system.
In accordance with one aspect of this invention, there is provided a printing system including a cylinder containing a thin film on which is formed an image to be printed and impression means for-carrying a media member in contact with the thin film.